My own thinking is that chapter 1 enemies are mostly melee and the 2H weapons require too much strength and agility (if sharpened) for use until higher levels so 1H and shield works best - possibly with a crush specialty. Later on 2H comes into its own when enemies attack with magic and doing major damage fast can outweigh blocking physical attacks with the shield e.g. with a certain shaman.
From my own experience, I would agree with this analysis. Don't forget that your DK's armour slowly improves throughout the game anyway, and there are many useful items that can increase armour rating even without a shield for your other character.
There are points in the game when kicking a lot of damage fast is VERY important <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Nearly forgot - there are a lot of really good 2H slashing weapons out there, so I went slashing over crushing.